Atomic clocks have been developed since more than 50 years, following fundamental scientific progress and developments in the field of quantum mechanics and microwave spectroscopy. Atomic clocks are based on microwave emission as a result of electronic transitions between the electronic energy levels of atoms. In practice alkali metals, in particular rubidium (Rb) and cesium (Cs), are the preferred choice for the interrogation due to their relative simple electron configuration and their high vapor pressure at relatively low temperatures. On the other hand a major challenge is related to the high reactivity of these alkali metals with oxygen and water. For this purpose the alkali metal is normally confined under a well-controlled atmosphere is a small cavity, called a vapor cell.
Vapor cell technology and control systems have made huge progress in the field of atomic clocks. It that field, the main aim is to improve the degree of accuracy and also the stability of the frequency signals delivered by the atomic clocks.
In 2004 the National Institute of Standards and Technology (NIST) presented the first chip-scale atomic clock: S. Knappe et al., “A Micro fabricated atomic clock”, Appl. Phys. Lett. 85, p. 1460, 2004. This chip-scale atomic clock had a volume of less than 10 mm3 and a power dissipation of 125 mW. This achievement led to the possibility to develop atomic clocks for portable and battery-driven devices. Since then chip-scale atomic clocks have gained a worldwide interest for a wide range of industrial applications.
Since the availability of MEMS fabrication techniques, chip-scale atomic clocks have been developed comprising vapor cells having internal gas volumes in the range of 1-10 mm3. Advantage of MEMS fabrication techniques is that thanks to the small size of the vapor cells hundreds or even thousands of vapor cells can be fabricated at once on a single silicon wafer, which cuts drastically the fabrication costs.
One of the issues arising from such very small volume vapor cells is that the size of a cross-section of the cavity in a plane perpendicular to the optical beam is close to the diameter of the optical beam itself, usually a laser beam, used to interrogate optically the alkali vapor cell by spectral absorption. Hence metallic alkali which is not in vapor phase may aggregate on a surface of the cavity in the optical path of the optical beam causing a well-known light-shift of the atomic frequency due to variation of light intensity induced by the presence of non-gaseous alkali metal which is typically in the form of alkali metal liquid droplets. The presence of non-transparent material within the light path decreases the transmission of the light through the cell by scattering or absorption. While this phenomenon is generally unwanted, a much more critical problem arises if the size, the shape or the location of the non-transparent areas within the light path is modified during the operation of the cell. Such uncontrolled variations lead to light shift effect.
The problem of the interference of metallic alkali, for example rubidium, within the optical path of an alkali vapor cell is well known and it is necessary to avoid and prevent it as much as possible. In traditional larger bulk cells this can be realized by localizing the alkali metal in a convenient location, for example by creating a cold area in a heated vapor cell. This approach is well known and is described for example in: McGuyer et al. “Atomic physics with Vapor-Cell Clocks”, p. 153, Dissertation, Princeton University, June 2012. Herein is described for example that the excess of alkali metal in the vapor cell required to produce a vapor introduces scattering and absorption, and also that these effects can be reduced by confining this excess in a more desirable portion in the cell through selective heating and cooling.
In a miniaturized vapor cell of a few cubic millimeters or smaller, precise localization of the metallic alkali outside the optical beam path is not easily realized in this way. Creating a thermal gradient allowing a well-defined localization within distances of 100 micrometers implies a complex set-up and a slow process which is not easily achievable at a wafer scale level and it will increase fabrication costs considerably.
Furthermore, during storage, when the cell temperature experiences an uncontrolled temperature gradient, the alkali metal can migrate to an unwanted location by the same mechanism. With larger volume clock systems, without stringent power consumption limitations, one could imagine to create a temperature gradient including a colder spot within the vapor cell, this ensuring the relocation of the alkali metal. Energy consumption is of paramount importance in miniaturized cells designed for portable applications. The thermal insulation of the heated vapor cell must be nearly perfect and any cold spot would be the source of an inacceptable power loss. Thus, a problem caused by a thermal gradient during storage might result in a permanently dysfunctional device.
Miniaturized cells comprising two reservoirs have also been reported in the past. In this approach the alkali metal is placed in a first chamber connected by a narrow opening to a second chamber where metallic vapor is diffused to as described in US 2011187464 (A1) and also in: R. Straessle et al. “Microfabricated alkali vapor cell with anti-relaxation wall coating”, Applied Physics Letters, vol. 105, nr.4, p. 043502, July 2014.